Thawing method

ABSTRACT

To provide a thawing method wherein a frozen commodity can be thawed, in a short time, from the inside with no unevenness. The thawing method, which thaws a frozen commodity by high frequency heating, includes: a first high frequency heating step in which the frozen commodity is high frequency heated to the vicinity of a melting temperature; a temperature detection step which measures the temperature of the frozen commodity during the first high frequency heating step and detects that the measured temperature has reached the melting temperature; a steam supplying step which starts a steam supply when the melting temperature is detected in the temperature detection step, thus forming a film of dew condensation on the surface of the frozen commodity; and a second high frequency heating step in which the frozen commodity is high frequency heated after the start of the steam supplying step.

TECHNICAL FIELD

The present invention relates to a thawing method which thaws a frozencommodity by high frequency heating.

BACKGROUND ART

An electronic microwave oven which heats a food commodity by highfrequency waves (microwaves) has been widely popularized. Generally,with the existing electronic oven, cooking is carried out by an emissionof high frequency waves into a heating chamber in accordance with thefood commodity to be heated. This kind of electronic oven may be usedfor the purpose of thawing a food commodity which has been frozen.

In the case of the conventional electronic oven, however, when heatingthe frozen commodity, as heating is carried out simply by an emission ofa high frequency waves, a problem has existed wherein a large unevennessin a heating temperature occurs depending on the size and theconfiguration of the food commodity, meaning that a uniform thawingcannot be carried out. For example, in the event that the configurationof the food commodity includes a corner, the corner is thawed firstwhile another portion remains frozen, resulting in a portion which isthawed and a portion which remains frozen. In the event that thiscondition occurs, a large quantity of high frequency waves is absorbedfirst by the thawed portion meaning that, in the event that the highfrequency heating is continued, the temperature difference between thecorner and the other portion becomes increasingly larger.

Also, particularly in the case of a large food commodity, there is anincreased tendency with high frequency heating for the surface of thefood commodity to be thawed while the inside remains frozen. At thispoint, it has happened that, in the event that high frequency heating iscarried out for a long time in order to thaw the inside of the foodcommodity, the temperature difference between the surface and the insideof the food commodity increases, with the result that the surface of thefood commodity has reached an overheated condition and has been cooked.Furthermore, although an electronic oven has been disclosed in PatentDocument 1 wherein, after a magnetron is driven and the food commodityis high frequency heated, a cooling fan device is driven to supply air,while the air supply has a notable effect on a temperature uniformitywith regard to the surface of the food commodity, it does not go so faras to eliminate a heating unevenness with regard to the inside of thefood commodity.

Patent Document 1: JP-A-9-101035

DISCLOSURE OF THE INVENTION Problems that the Invention is to Solve

The invention has been made in view of the existing circumstances, andan object of the invention is to provide a thawing method which iscapable of thawing a frozen commodity, in a short time, from the insidewith no unevenness.

Means for Solving the Problem

The thawing method of the invention is a thawing method by which afrozen commodity is thawed by means of high frequency heatingcomprising: a first high frequency heating step in which the frozencommodity is high frequency heated to the vicinity of a meltingtemperature; a temperature detection step which measures the temperatureof the frozen commodity during the first high frequency heating step anddetects that the measured temperature has reached the meltingtemperature; a steam supplying step which starts a steam supply when themelting temperature is detected in the temperature detection step, thusforming a film of dew condensation on the surface of the frozencommodity; and a second high frequency heating step in which the frozencommodity is high frequency heated after the start of the steamsupplying step. By means of this configuration, the frozen commodity canbe heated from the inside, in a short time, in the first high frequencyheating step. Also, the frozen commodity, over the whole surface ofwhich a film of dew condensation has been formed by the steam supplystep, is high frequency heated once more in the second high frequencyheating step, whereby the frozen commodity can be thawed from the wholesurface with no unevenness. Additionally, as the film of dewcondensation is formed over the whole surface of the frozen commodity,the frost adhering to the surface of the frozen commodity can beremoved.

Also, the thawing method of the invention includes a weight evaluationstep which evaluates the weight of the frozen commodity based on atemperature increase rate of the frozen commodity measured in thetemperature detection step, wherein the heating time of the first highfrequency heating step is set in accordance with the weight evaluationresult from the weight evaluation step. By means of this configuration,the frozen commodity can be high frequency heated for a time appropriateto the weight of the frozen commodity, thereby preventing an overheatingand an insufficient thawing.

Also, the thawing method of the invention includes a weight evaluationstep which evaluates the weight of the frozen commodity based on atemperature increase rate of the frozen commodity measured in thetemperature detection step, wherein the steam supplying time of thesteam supplying step is set in accordance with the weight evaluationresult from the weight evaluation step. By means of this configuration,as it is possible to supply an amount of steam appropriate to the weightof the frozen commodity to the inside of the heating chamber, meaningthat no excess steam remains inside the heating chamber, it is possibleto prevent condensation, and it is also possible to prevent an adherenceof excess moisture to the frozen commodity.

Also, the thawing method of the invention includes a steam exhaust stepwhich reduces an amount of steam inside a heating chamber, in which thefrozen commodity is placed, after the steam supplying step. By means ofthis configuration, as the amount of steam inside the heating chamber isreduced even in the event that an excess of steam remains inside theheating chamber, condensation can be prevented, and the adherence ofexcess moisture to the frozen commodity can also be prevented.

Furthermore, in the thawing method of the invention, the temperaturedetection step, while scanning an infrared ray sensor whichsimultaneously measures a plurality of points, detects that the averagevalue of the surface temperature of the measured frozen commodity hasreached the melting temperature. By means of this configuration, theaccuracy of the temperature detection can be increased.

ADVANTAGE OF THE INVENTION

According to the invention, it is possible to thaw a frozen commodity,in a short time, from the inside with no unevenness.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a front view showing an opening and closing door of a heatingcooking apparatus in an opened condition, to illustrate one embodimentof the invention;

FIG. 2 is a control block diagram of the heating cooking apparatus;

FIG. 3 is a front view showing an example of a key arrangement of anoperating panel;

FIG. 4 is an illustration representing a basic principle of the steamgeneration of the heating cooking apparatus;

FIG. 5A is an external view (top surface) of a heating block, while FIG.5B is an external view (rear surface) of the heating block;

FIG. 6 is a schematic perspective view of a rear corner of a heatingchamber seen from the outside;

FIG. 7 is an illustration of a configuration example of a damperprovided in a duct;

FIG. 8 is an illustration of another configuration example of the damperprovided in the duct;

FIG. 9 is a flowchart which illustrates a thawing method according tothe embodiment;

FIG. 10 is a time chart which illustrates the thawing method accordingto the embodiment;

FIG. 11A is a perspective view showing a condition of a temperaturemeasurement of a heated object by a scanning of an infrared sensor,while FIG. 11B is a graph showing a result of the temperaturemeasurement;

FIG. 12 is a graph showing a temperature distribution of an L lineposition in FIG. 11B when a scan by an infrared sensor is carried outconsecutively; and

FIG. 13 is a graph showing a relationship between a heating time and ameasured temperature depending on a difference in quantity.

DESCRIPTION OF REFERENCE NUMERALS AND SIGNS

1 Exhaust opening

2 Air supply opening

6 Cooling fan

11 Heating chamber

13 Magnetron

15 Steam generator

17 Heater means

27 Water supply tank

32 Heated object temperature sensor

33 Circulation fan

35 Convection heater

45 Heating block

45 a Heating surface

46 Support plate

46 a Opening

47 Heating cooking apparatus casing bottom

47 a Opening

48 Vaporization tray

48 a Water storage depression

52 Sheathed heater

61 Heating block main body

62 Rib

62 a Inner surface

69 Water supply means

74 Intake pipe

75 Nozzle

76 Feed connecting port

77 Water pump

78 Discharge nozzle

79 Water supplying pump

100 Heating cooking apparatus

BEST MODE FOR CARRYING OUT THE INVENTION

Although the invention relates to a method of thawing a frozen commodityby means of high frequency heating, in this embodiment, an example ofthawing a frozen commodity using a heating cooking apparatus will bedescribed.

FIG. 1 is a front view showing an opening and closing door of theheating cooking apparatus in an opened condition, to illustrate oneembodiment of the invention, FIG. 2 is a control block diagram of theheating cooking apparatus, and FIG. 3 is a front view showing an exampleof a key arrangement of an operating panel.

First, a configuration of a heating cooking apparatus 100 will bedescribed. As shown in FIG. 1, the heating cooking apparatus 100includes, as basic components, a magnetron 13 which generates a highfrequency wave (microwave), a steam generator 15 which generates steam(steam) inside a heating chamber 11, and an operating panel 91 (refer toFIG. 3) for the purpose of starting a thawing step of a frozencommodity.

The high frequency waves from the magnetron 13 are dispersed around thewhole of the heating chamber 11 by a rotary driven electrical waveagitating antenna 23. Also, the configuration is such that water issupplied to the steam generator 15 from a water supply tank 27, which isprovided in a side of the heating chamber 11.

The steam generator 15 is configured in such a way as to generate asteam at the rear of the bottom inside the heating chamber 11. A heatedobject temperature sensor 32, such as an infrared ray sensor, isattached to the rear wall of the heating chamber 11, wherein thetemperature of a heated object (frozen commodity) is measured via adetecting hole 18 which is provided in a wall of the heating chamber 11.

The heating chamber 11 is formed inside a main body casing 10, which isa front-opening box shape, wherein the opening and closing door 41,which is equipped with a translucent window 41 a, and which opens andcloses an aperture for removing the heated object, is attached to thefront of the main body casing 10 in such a way as to be able to open andclose.

Also, an air supply opening 2 is provided at the top of one side wall ofthe heating chamber 11, while an exhaust opening 1 is provided at thebottom of the other side wall. The air supply opening 2 and the exhaustopening 1 are openings for the purpose of carrying out the supply andexhaust of air between the outside of the heating chamber 11 and theinside of the heating chamber 11, being formed as a large quantity ofpunch holes. The positions in which the air supply opening and theexhaust opening are provided are not limited to those describedheretofore. That is, for example, it is acceptable for the air supplyopening and the exhaust opening to be provided at either the top orbottom of a side wall, and it is also acceptable for the air supplyopening and the exhaust opening to be provided in the same side wall.

Also, an internal air heater 37, which comprises a circulation fan 33,which agitates and circulates the air inside the heating chamber 11, anda convection heater 35, which heats the air which circulates inside theheating chamber 11, is attached in the heating cooking apparatus 100 ofthis embodiment. Furthermore, heater means 17 (refer to FIG. 2), whichcomprises a mica heater, sheathed heater or the like, is provided in awall, such as the top or a side, inside the heating chamber 11. Anoperation of each of the parts is carried out according to a controlinstruction from a controller 39, which is equipped with amicroprocessor (refer to FIG. 2).

Also, a start key 93, which instructs a start of heating (thawing), isprovided on the operating panel 91 which, shown as one example in FIG.3, is provided on the opening and closing door 41. Knobs 96, which areused to set a heating temperature and a heating time and the like, arealso provided on the operating panel 91. When carrying out thawing of afood item, the configuration is such that, after operating a “thaw” key,which is provided as one of two cooking menu keys 94 and 97, the startis instructed via the start key 93.

The controller 39, being supplied with power from a power source 40which is connected to a commercial power source, controls a distributionof power to each of the magnetron 13, the heater means 17, the steamgenerator 15 etc. so that a heating power thereof does not exceed apermissible power value. Also, the controller 39 time controls each partby means of a timer 101.

The magnetron 13 is disposed in a space in the bottom of the heatingchamber 11. Also, the antenna 23 is provided in a position in theapproximate center of the bottom of the heating chamber 11, in which itreceives the high frequency waves generated by the magnetron. Themagnetron 13 and the antenna 23 can be provided not only in the bottomof the heating chamber 11, but also in another surface of the heatingchamber 11. A configuration comprising a turntable in place of theantenna 23 is also acceptable.

Next, a description will be given of the steam generator 15, withreference to FIG. 4. An exploded perspective view of the steam generator15 is shown in FIG. 4.

The steam generator 15 includes a vaporization tray 48, which is of anelongated configuration and includes a water storage depression 48 a ina top surface, and heating blocks 45, to be described hereafter, for thepurpose of heating the vaporization tray 48, wherein the vaporizationtray 48 and the heating blocks 45 are affixed to a casing bottom 47 ofthe heating cooking apparatus 100. Specifically, the two heating blocks45 are temporarily screwed to a support plate 46, which includes anopening 46 a which exposes a heating surface 45 a, and affixed to therear of the casing bottom 47. Meanwhile, wherein an opening 47 a isformed in the frame bottom 47 to contain a projection of the waterstorage depression 48 a of the vaporization tray 48, the vaporizationtray 48 is affixed by means of projecting the water storage depression48 a downwards through the opening 47 a. Also, the support plate 46 towhich the heating blocks 45 are attached is affixed to the rear of theframe bottom 47 from below. By this means, the bottom of thevaporization tray 48 comes into contact with the heating surfaces 45 aof the heating blocks 45, thus enabling transmission of a heat emittedby the heating blocks 45 to the vaporization tray 48. It is preferablethat the support plate 46 and the vaporization tray 48 are formed of ametal material with a high heat conductivity, such as aluminum.

FIG. 5 shows an external perspective view of a heating block. FIG. 5(a)is a top surface while FIG. 5(b) is a rear surface.

The heating block 45 is a lightweight, highly heat conductive aluminumdie cast. The heating block 45 comprises a U-shaped sheathed heater 53embedded inside a main body 61 as a heater which heats the steamgenerating portion, wherein two ribs 62 protrude in the vicinity of thesides of the top surface, parallel with the straight portion of thesheathed heater 53, along the whole of the longitudinal direction of themain body 61, and opposing inside surfaces 62 a of the ribs 62 have aslanting curved surface which matches the curved surface of the bottomcorners of the vaporization tray 48. The curved surface configurationincreases the air tightness with the vaporization tray 48, thusimproving the heat transmission. The heating block 45 includes a screwhole in four corners, wherein the four corners are screwed so that theribs 62 are in contact with the inside of the opening 46 a of theplate-like member 46. In this embodiment, the two heating blocks 45 areprovided, through the plate-like member 46, in the proximity of thecenter of the vaporization tray 48. The configuration is not limited tothe one heretofore described, as it is acceptable to provide one, orthree or more, heating blocks. A configuration is also acceptablewherein heating means such as a sheathed heater is disposed along alongitudinal direction on the bottom of the vaporization tray 48.

As the heating block 45 is formed of a highly heat conductive aluminumdie cast, the heat from the sheathed heater 53 can be transmitted at ahigh efficiency to the water storage depression 45 a. A configurationand attachment position of the sheathed heater 53 and the like can bechanged as appropriate in response to a necessary amount of heating, aninstallation space inside the casing of the heating cooking apparatus100, and the like. It is acceptable that the sheathed heater 53 isanother kind of heater, such as a wire heater or a ceramic heater.

The configuration of the steam generator 15 is such that steam issupplied from the bottom of the heating chamber 11 and the steam isefficiently dispersed inside the heating chamber 11. Also, even in theevent that dirt adheres to the vaporization tray 48, it can be easilyremoved. That is, although the calcium, magnesium, chloride compoundsand the like contained in the moisture may be concentrated, depositedand attached to the bottom of the water storage depression 45 a duringthe steam generating step, as the water storage depression 45 a isexposed inside the heating chamber 11, this kind of dirt can be easilycleaned off by wiping the water storage depression 45 a with a cloth orthe like.

At this point, a water supply route from the water supply tank 27 shownin FIG. 1 to the steam generator 15 will be described with reference toFIG. 6. FIG. 6 is a schematic perspective view of a rear corner of theheating chamber 11 seen from the outside.

The water supply tank 27 is one which enables a supply of water to thevaporization tray 48 by an attachable and removable insertion of anozzle 75, which is connected to an intake pipe 74, into a feedconnecting port 76 affixed on the heating cooking apparatus 100 side,wherein the water is stored inside a transparent casing which enables avisual checking of a water level.

A water supplying pump 79 is driven in accordance with an instructionfrom the controller 39 (refer to FIG. 2) to suck up the water from thewater supply tank 27 through the feed connecting port 76, therebyfeeding, via a water pump 77, a prescribed amount of water to thevaporization tray 48 from a discharge nozzle 78.

With the water from the water supply tank 27 having been supplied to thevaporization tray 48, the bottom of the vaporization tray 48 is heatedby an emission of heat from the sheathed heater 53 of the heating block45, whereupon the steam generator steams the water inside thevaporization tray 48.

Next, a description will be given of a configuration wherein a coolingair is supplied to the inside of the heating chamber 11 for the purposeof efficiently exhausting the steam inside the heating chamber 11.

Electrical parts such as a magnetron and a circuit board are installedon the bottom of the heating cooking apparatus 100, wherein the air froma magnetron cooling fan 6 is blown against each electrical part, therebycooling it. At this time, the supply of the cooling air from the bottomof the heating cooking apparatus 100 to the inside of the heatingchamber 11 is made possible through ducts 4 and 5 provided in aperimeter of the heating chamber 11. For example, as shown in FIG. 7, adamper 3 is provided between the ducts 4 and 5, wherein the cooling airis supplied to the heating chamber 11, or exhausted to the outside fromthe exhaust opening 6, by means of a displacement of the damper 3. Aconfiguration is also acceptable wherein a shutter which blocks apassage between the ducts 4 and 5 is simply provided in place of adamper. Furthermore, a configuration is also acceptable wherein, asshown in FIG. 8, a damper 7, which blocks the exhaust opening 1 of theheating chamber 11, is provided.

In this case, it is acceptable to provide a separate, dedicated coolingfan, as well as a magnetron cooling fan, as the cooling fan 6, whereinair is supplied to the heating chamber 11 at an optional timing,regardless of an existence or non-existence of the generation of thehigh frequency waves. According to the configuration, an excess steamwhich remains inside the heating chamber 11 can be effectivelyexhausted, and an amount (density) of steam inside the heating chamber11 can be reduced. Furthermore, by causing the circulation fan 33 torotate, the steam exhaust efficiency can be increased.

Next, one example of an implementation of the thawing method accordingto the invention, using the heating cooking apparatus 100, will bedescribed with reference to FIGS. 9 and 10. FIG. 9 is a flowchart whichillustrates the thawing method of this embodiment, while FIG. 10 is anexplanatory diagram which functionally illustrates the thawing method ofthis embodiment, wherein an example of a change in temperature in thecase of thawing 300 g of −18° C. frozen sliced meat (hereafter referredto as “frozen commodity M”) is shown.

First, the frozen commodity M which is to be thawed is placed onto aplate or the like, and put into the heating chamber 11, whereupon theopening and closing door 41 is closed. Then, after putting the “thaw”key on the operating panel 91 on, the start key is put on (step S0).

When the start key is put on, as well as the magnetron 13 generatinghigh frequency waves (microwaves), the heated object temperature sensor32 starts measuring the temperature of the frozen commodity M (step S2).Also, in step S2, the weight of the frozen commodity M is evaluated. Thedetails of the temperature measurement and the weight evaluation at thistime will be described hereafter. Furthermore, based on a result of theweight evaluation carried out in step S2, a time of the steam supplycarried out in step S6 and a time of the high frequency heating carriedout in step S8 are set.

By means of the high frequency heating in step S2, the frozen commodityM is heated from the inside. As, in general, high frequency waves have alow absorption and a large depth of penetration with respect to ice,when the frozen commodity M is high frequency heated, a heating effectworks not only on the surface of the frozen commodity M but also on theinside, thus having a benefit of accelerating the thawing. Then, bycontinuing the high frequency heating, the temperature of the frozencommodity M gradually rises, wherein one part of the surface of thefrozen commodity M, which has been in a frozen condition, begins tomelt.

Also, it is determined whether or not the surface temperature of thefrozen commodity M which is measured in step S2 has reached a prescribedvalue (step S4). In the event that the surface temperature of the frozencommodity M has reached the prescribed temperature (a meltingtemperature to be described hereafter), as well as the high temperatureheating being stopped, the supply of steam to the inside of the heatingchamber 11 by the steam generator 15 is started (step S6). While thehigh temperature heating is being carried out, the heated objecttemperature sensor 32 monitors the temperature of the frozen commodityM, finishing the temperature measurement when the steam supply isstarted.

The melting temperature which determines the timing of the start of thesteam supply is the temperature at which one part of the surface of thefrozen commodity M begins to melt, and is set at a predetermined valuebased on an experiment. The melting temperature is the surfacetemperature at a point when the temperature of the inside of the fooditem reaches the order of −5 to 0° C. (maximum ice crystal generationzone). For example, in a case of an example shown in FIG. 8, when onepart of the surface of the frozen commodity M begins to melt, themeasured temperature according to the heated object temperature sensor32 is 3 to 7° C., while the actual temperature of the frozen commodity M(an average temperature of the frozen commodity M from the surfacethrough the whole of the inside) is −5 to 0° C. That is, in this case,the melting temperature which determines the timing of the start of thesteam supply is 3 to 7° C. The surface temperature and the actualtemperature of the frozen commodity M shown in FIG. 8 are based on theresult of a previously conducted experiment. In the case of the exampleshown in FIG. 8, the steam supply is started approximately two minutesafter the start of the high frequency heating.

In step S6, the high frequency heating is stopped along with the steamsupply being started. As a result, as there is no concentratedabsorption of the high frequency waves by the part of the frozencommodity M which has started to melt, uneven thawing can be reliablyprevented. Also, while the steam is being supplied, a localizedtemperature unevenness of the frozen commodity M is reduced by the heattransmission. Furthermore, an amount of power consumption during thesteam supply can be suppressed. It is also acceptable that the steamsupply and the high frequency heating are carried out in parallel. Inthis case, in order to prevent uneven thawing, it is preferable that thehigh frequency heating is carried out at a low power. By this means, areduction in a temperature increase rate due to condensation can besuppressed, wherein swift thawing can be carried out by an effective useof a cooking time. As the amount of steam inside the heating chamber 11increases in step S6, it is easy for condensation to form on the lowtemperature surface of the frozen commodity M. Then, after a certaintime has elapsed from the start of the steam supply, the whole of thesurface of the frozen commodity M is covered in a film of condensation.When this kind of condition occurs, the steam supply is stopped. As thetime until the surface of the frozen commodity is covered in a film ofcondensation varies depending on a volume and a heat capacity predictedfrom the weight of the frozen commodity, as well as an initialtemperature of the frozen commodity and the like, it is preferable thatthe steam supply time is adjusted (set) beforehand in accordance withthe weight and the like of the frozen commodity. The controller 39 setsa control timing for each part heretofore described, in accordance withthe elapsed time of the heating time and the like measured by the timer101. After the steam supply is started in step S6, it is determinedwhether or not a preset prescribed time has elapsed (step S7). In theevent that the prescribed time has elapsed, the process moves on to stepS8, while in the event that it has not elapsed, the steam supply iscontinued. Also, in the case of the example shown in FIG. 8, the steamsupply time is approximately one minute.

Next, after the film of condensation has been formed on the whole of thesurface of the frozen commodity M, as well as the high frequency heatingbeing restarted, the steam remaining inside the heating chamber 11 isexhausted (step S8). As the whole of the frozen commodity M is coveredin the film of condensation, in the event that the high frequency wavesare applied to the frozen commodity M in this condition, the absorptionrate of the high frequency waves becomes uniform over the whole of thefrozen commodity. Consequently, by heating the frozen commodity M, whichis covered in the film of condensation, at a high frequency, as well asenabling uniform heating with no unevenness, it is possible to remove afrost from the surface of the frozen commodity M. It is preferable thatthe power of the high frequency waves at this time is set lower than inthe case of step S2, thereby suppressing an abrupt heating. In step S6,however, after the steam is supplied, the steam remains inside theheating chamber 11. The steam is efficiently exhausted to the outside ofthe heating chamber 11 by using the exhaust opening 1 and the air supplyopening 2. That is, together with the air outside the heating chamber 11being drawn in through the air supply opening 2, the air inside theheating chamber 11 is exhausted through the exhaust opening 1. By thismeans, the amount (density) of steam included in the air inside theheating chamber 11 can be reduced wherein, as no excess steam remainsinside the heating chamber 11, an occurrence of unnecessary condensationon the wall and the like of the heating chamber 11 can be prevented, andan adherence of excess moisture to the frozen commodity M can also beprevented. By this means, it is possible to fill the inside of theheating chamber 11 with steam only when the condensation is necessary.Also, by setting in advance an appropriate steam supply time based onthe heretofore described weight evaluation result in step S2, only thenecessary amount of steam is supplied, meaning that the steam exhauststep can be omitted.

Next, the controller 39, with reference to the time measured by thetimer 101, determines whether or not the high frequency heating time setin step S2 has elapsed (step S10). In the event that it is determined instep S10 that the high frequency heating time has elapsed, the highfrequency heating and the like is finished, and the thawing step isfinished (step S12). Whereas an actual temperature (a completiontemperature) of the frozen commodity M at the time of finishing thethawing step is, for example in the case of the example shown in FIG. 8,in the order of 0 to 10° C., the surface temperature (the result of themeasurement by the heated object temperature sensor 32) of the frozencommodity M at the time is approximately 15° C. Although the correlationbetween the surface temperature and the actual temperature of the frozencommodity differs depending on the type, weight, configuration and thelike (whether it is a lump or a thinly sliced article etc.) of thefrozen commodity, the actual temperature can be known from the surfacetemperature based on the result of the experiment conducted in advance.

With reference to FIG. 11, a description will now be given of themeasurement of the temperature of the frozen commodity M by the heatedobject temperature sensor 32. Although the frozen commodity M is placedinside the heating chamber 11, it is unclear in what position on thebottom of the heating chamber 11 the frozen commodity is placed at thestart of the thawing. For this reason, the position of the frozencommodity M is calculated from a temperature distribution inside theheating chamber 11 obtained from the heated object temperature sensor32. That is, as shown in FIG. 11(a), the heated object temperaturesensor 32, while simultaneously detecting the temperature of a pluralityof points (n points) at one time, scans in the direction of the arrow inthe drawing by oscillating the heated object temperature sensor 32itself, thereby detecting the temperature at a plurality of measurementpoints (m points in the scanning direction) inside the heating chamber11. Consequently, in one scan, temperature detection is possible at then×m measuring points shown in FIG. 11(b).

As can be clearly seen from the temperature distribution in the heatingchamber 11 measured by one scan of the heated object temperature sensor32 shown in FIG. 11(b), normally, the temperature of the place in whichthe frozen commodity M exists is detected to be a different temperatureto that of the other areas, thus enabling the position of the frozencommodity M inside the heating chamber 11 to be identified. That is,when temperature measurement is started, as the temperature in oneparticular area is detected to be low in comparison with the temperature(normal temperature) of the bottom of the heating chamber 11, theposition of the frozen commodity M can be identified.

FIG. 12 shows the temperature distribution of an L line position in FIG.11(b) when a scan by the heated object temperature sensor 32 is carriedout consecutively a plurality of times. In FIG. 12, a temperaturereduction peak position of the temperature distribution when thetemperature particularly changes within the range of one scancorresponds to the position of the frozen commodity M on the L line inFIG. 11(b). Consequently, the position of the frozen commodity M in theheating chamber 11 can be calculated from the peak position of thetemperature distribution. Then, by tracing the temperature correspondingto the position of the frozen commodity M back to the initial heatingtime or the temperature measurement starting time, the initialtemperature of the frozen commodity M is determined.

When the initial temperature has been determined, a temperature increaserate ΔT of the frozen commodity M is calculated from the gradient of theline (the dotted line in FIG. 12) which connects each peak of thetemperature distribution curve in FIG. 12. A quantity of the frozencommodity M is estimated from the temperature increase rate ΔT. That is,as shown in FIG. 13, the quantity is estimated by using the fact thatwhen heating two frozen commodities M1 and M2 of a different weightunder the same conditions at the same initial temperature, thetemperature increase rate ΔT differs according to the weight. Forexample, whereas in a case of heating the frozen commodity M1, which hasa small quantity, the temperature increase rate is ΔTL, in a case ofheating the frozen commodity M2, which has a large quantity, thetemperature increase rate is ΔTM, which is smaller than ΔTL.

In this way, when the determining of the initial temperature of thefrozen commodity M, and the estimation of the quantity of the frozencommodity M from temperature increase rate ΔT, have been completed, thehigh frequency heating time is set in accordance with the calculatedtemperature increase rate ΔT. For example, it is calculated as K1/ΔT (K1is a constant). Furthermore, at this point, the setting of the maximumheating time corresponding to the quantity of the frozen commodity isalso carried out. In the case of a subsequent heating step, when thetotal heating time exceeds the maximum heating time, control is carriedout whereby the heating step is compulsorily finished. By this means, anoverheating is prevented and the safety of the apparatus can bemaintained.

In this way, when measuring the surface temperature of the frozencommodity M in step S2, the accuracy of the temperature measurement isincreased by scanning the heated object temperature sensor 32, whichsimultaneously measures a plurality of points, and calculating theaverage value for the area which corresponds to the frozen commodity M.

Although, thus far, a description has been given of the case in whicheach step (the high frequency heating in step S2 and S8, the steamsupply in step S6) is started or finished in accordance with the factthat the prescribed time based on the weight of the frozen commodity haselapsed, it is also acceptable to determine the timing of each processby another method. For example, it is acceptable that the timing of eachprocess is the time, wherein the surface temperature of the frozencommodity M is constantly measured, when the surface temperature reachesthe prescribed value, or, wherein a table is prepared in advance to showthe type, weight, configuration and the like of the frozen commodity,and the surface temperature change sample data obtained by changing thetime for which each process is conducted, the time when the prescribedtime based on the table has elapsed. Furthermore, it is also acceptableto determine the timing of each process by constantly measuring thesurface temperature of the frozen commodity, and comparing the actualvalue with the temperature shown in the aforementioned table.

As has been heretofore described, according to the thawing method ofthis embodiment, the frozen commodity can be swiftly thawed from theinside by high frequency waves. Also, the frozen commodity, over thewhole surface of which a film of condensation has been formed by thesteam supply, is high frequency heated once more, whereby the frozencommodity can be uniformly thawed with no unevenness. Additionally, asthe film condensation is formed over the whole surface of the frozencommodity, the frost adhering to the surface of the frozen commodity canbe melted and removed. Furthermore, in the thawing step, it is possibleto gradually thaw from the inside without abruptly raising the surfacetemperature of the frozen commodity.

Also, as the weight is evaluated based on the result of the temperaturemeasurement using the heated object temperature sensor 32, the frozencommodity can be high frequency heated for a time appropriate to theweight of the frozen commodity, thereby preventing an overheating and aninsufficient thawing, and enabling the carrying out of an extremelyefficient thawing step.

Second Embodiment

Although the thawing method of this embodiment is approximatelyidentical to the thawing method described in the first embodiment, thetiming of the start of the steam supply shown in FIG. 9 is different. Inthis embodiment, regardless of whether or not the measured surfacetemperature of the frozen commodity M has reached the meltingtemperature, in the event that the prescribed time set in accordancewith the weight evaluation result in step S2 has elapsed, the steamsupply is started. That is, even in the case where it is determined thatthe melting temperature has been reached in step S4, in the event thatthe aforementioned prescribed time has not elapsed, the high frequencyheating is continued without the steam supply being started. On thecontrary, even in the case where it is not determined that the meltingtemperature has been reached in step S4, in the event that theaforementioned prescribed time has elapsed, the steam supply is startedwhile the high frequency heating is stopped.

By this means, for example, in the case of thawing a frozen commoditywhich has a projection and the like, even when the average value of thesurface temperature of the frozen commodity is measured to be higherthan the actual value as the temperature of one part of the frozencommodity rises, the frozen commodity can be high frequency heated foran appropriate time.

Although the invention has been described in detail and with referenceto a specific embodiment, it is apparent to those skilled in the artthat a variety of changes and modifications can be added withoutdeparting from the spirit and scope of the invention.

The present application is based on a Japanese patent application filedon Nov. 13, 2003, application number 2003-383421, the contents of whichare included as reference herein.

INDUSTRIAL APPLICABILITY

The thawing method of the invention has a benefit of thawing the frozencommodity from the inside, with no unevenness, in a short time, therebybeing useful in the case of thawing the frozen commodity by highfrequency heating and in a like case.

DRAWING(S)

[FIG. 2]

-   91 Operating panel-   40 Power source-   101 Timer-   39 Controller-   13 High frequency wave generator-   6 Cooling fan-   17 Heater means-   15 Steam generator-   53 Steam generator heating heater-   37 Internal air heater-   35 Convection heater-   33 Circulation fan-   32 Heated object temperature sensor    [FIG. 7]-   Exhaust-   Exhaust-   Bottom    [FIG. 8]-   Exhaust-   Bottom    [FIG. 9]-   S0 Start-   S2 High frequency heating-   Temperature measurement-   Weight evaluation-   S4 Melting temperature reached?-   S6 Steam supply-   S7 Prescribed time elapsed?-   S8 High frequency heating-   (steam exhaust)-   S10 Prescribed time elapsed?-   S12 Finish    [FIG. 10]-   Cooking start-   Microwaves-   Steam-   Temperature-   First high temperature heating step-   Microwaves-   Monitor with heated object temperature sensor until prescribed    temperature reached-   Steam supplying step-   Steam-   Melting temperature-   Maximum ice crystal generation zone-   Second high temperature heating step-   Microwaves-   Surface temperature-   Actual temperature-   Elapsed time-   Cooking finish    [FIG. 11]-   (a)-   Scan    [FIG. 12]-   Detected temperature-   Position of frozen commodity-   One scan width-   Scan origin position-   Overall scan length    [FIG. 13]-   Measured temperature-   Initial temperature-   Heating time-   Small quantity (M₁)-   Large quantity (M₂)-   Temperature increase rate ΔT_(L)>ΔT_(M)

1. A thawing method by which a frozen commodity is thawed by means ofhigh frequency heating comprising: a first high frequency heating stepfor high frequency heating the frozen commodity to the vicinity of amelting temperature; a temperature detection step for measuring thetemperature of the frozen commodity during the first high frequencyheating step and detects that the measured temperature has reached themelting temperature; a steam supplying step for starting a steam supplywhen the melting temperature is detected in the temperature detectionstep, thus forming a film of dew condensation on the surface of thefrozen commodity; and a second high frequency heating step for highfrequency heating the frozen commodity after the start of the steamsupplying.
 2. A thawing method according to claim 1, including a weightevaluation step for evaluating the weight of the frozen commodity basedon a temperature increase rate of the frozen commodity measured in thetemperature detection step, wherein the heating time of the first highfrequency heating step is set in accordance with the weight evaluationresult from the weight evaluation step.
 3. A thawing method according toclaim 1, including a weight evaluation step for evaluating the weight ofthe frozen commodity based on a temperature increase rate of the frozencommodity measured in the temperature detection step, wherein the steamsupplying time of the steam supplying step is set in accordance with theweight evaluation result from the weight evaluation step.
 4. A thawingmethod according to claim 1, including a steam exhaust step for reducingan amount of steam inside a heating chamber, in which the frozencommodity is placed, after the steam supplying step.
 5. A thawing methodaccording to claim 1, wherein the temperature detection step, whilescanning an infrared ray sensor which simultaneously measures aplurality of points, detects that the average value of the surfacetemperature of the measured frozen commodity has reached the meltingtemperature.